(1) Field of the Technology
The present technology relates to a developing device and an image forming apparatus for visualizing an electrostatic latent image formed on a photoreceptor etc. with a developer, in particular relating to a developing device and an image forming apparatus including an agitating mechanism for dual-component developer composed of toner and carrier.
(2) Description of the Prior Art
The developing device using a dual-component developer in an image forming apparatus such as a digital multifunctional machine or the like incorporates a developing roller opposing a photoreceptor, an agitating roller and a toner supply and agitation roller, all being arranged rotatably in the developing hopper. Formed over the toner supply and agitation roller is a toner supply port, over which a toner supply container is attached.
An excess of developer that was separated from the developing roller by the layer thickness-regulating member inside the developing hopper passes by a flow-guide plate arranged nearby and is returned to the upside of the agitating roller. This surplus developer is mixed and agitated with the supplied toner and then fed once again to the developing roller. Further, there are many cases that the flow-guide plate is formed with partitioning ribs in order to prevent occurrence of uneven distribution of the developer inside the developing hopper.
In the prior art, since partitioning ribs 347 formed on the flow-guide plate do not move as shown in FIG. 1, the surplus developer that was separated by an unillustrated layer thickness-regulating member will not move in the longitudinal direction of the developing roller, designated at 341 and is returned sliding over a flow-guide plate 346 to the upside of an unillustrated agitating roller. For this reason, agitation of the developer in the longitudinal direction of developing roller 341 relies on the rotation of the agitating roller alone, hence there occurred many cases where it takes long time for agitation or where a lack of agitation takes place.
As a countermeasure to deal with this problem, a patent document 1 (Japanese Patent Application Laid-open 2006-154235) discloses a technique in which the developer returned from the flow-guide plate and the toner supplied from above are made to pass through an AC magnetic field generator so as to improve agitating and mixing performance.
In recent high-speed digital multi-functional machines, there are cases that image unevenness occurs when printing of the same print pattern has been continuously performed. That is, in a dual-component developing device, the dual-component developer that is assumed to have a uniform toner concentration, is supplied to the developing roller using the agitating roller and the like so as to make the toner concentration uniform along the longitudinal direction of the developing roller. However, in the real situation, the amount of toner consumed from the developing roller is different depending on the print pattern, so that the toner concentration on the developing roller surface after development becomes different with respect to the aforementioned longitudinal direction. In this situation, when the toner concentration of the developer supplied to the developing roller is not uniform in the longitudinal direction, the toner concentration of the developer contributing to development resultantly becomes different across the length of the developing roller. This problem is particularly prone to occur for the developing roller that rotates at a high speed.
For example, when print patterns having a black solid area in the central part have been continuously printed, the toner concentration of the developer has become lowered in the center across the length of the developing roller, hence causing image unevenness such that the necessary print density cannot be obtained. As a result, there occurs a large difference in the toner concentration of the dual-component developer between before and after it is agitated and mixed in the agitating roller area, hence the toner concentration will remain uneven across the length of the developing roller even when the developer is supplied to the developing roller. Accordingly, the surplus dual-component developer is also different in toner concentration along the length of the developing roller when viewed microscopically.
Also, when print patterns having gradually changing printing ratios along the longitudinal direction of the developing hopper have been continuously printed, the toner concentration becomes different between the far and near sides across the length of the developing roller, hence making the images uneven or different in print density. As a result, there occurs a large difference in the toner concentration of the dual-component developer between before and after it is agitated and mixed in the agitating roller area, hence the toner concentration will similarly remain uneven across the length of the developing roller when the developer is supplied to the developing roller. Accordingly, the surplus dual-component developer is also different in toner concentration along the length of the developing roller when viewed microscopically.
However, the developing device disclosed in the aforementioned patent document 1 is constructed to focus on crushing toner aggregations, but the toner concentration across the length of the developing roller is not taken into consideration. Further, since this configuration takes such a structure that the developer is dropped through one place into the developing hopper, there has been the problem that it takes long time to make the toner concentration uniform.